1. Field of the Invention
The present invention relates to a projection apparatus of the type which includes an optical system for forming, on a photosensitive member, an image of an object illuminated by an illumination source wherein at least one lens in the lens system disposed in the optical path extending from the illumination source to the photosensitive member possesses a property of spectral transmission factor to compensate for the spectral sensitivity of the photosensitive member.
The term "lens" as used herein means a pure lens only and does not include those lenses which have a multilayer interference coating or the like applied thereon. The term "photosensitive member" should be understood to include a conventional photosensitive drum or sheet, various solid state image sensors such as CCD and image pickup tubes such as a vidicon.
The projection apparatus having the lens particularly mentioned above has many applications. For example, it may be used for a copying machine, facsimile equipment, a television camera etc. In this specification, the present invention will be described in detail in connection with a copying machine as one possible application for the projection apparatus.
2. Description of the Prior Art
In general, the photosensitive member used in a copying machine has a spectral sensitivity different from the spectral sensitivity of the human eye (relative visibility, relative luminosity or relative luminous efficiency). The composite spectral sensitivity resulting from the combination of the photosensitive member and the illumination source is also different from the relative visibility. Therefore, the copies obtained from the copying machine have different contrast from that of the original.
By way of example, let us consider the combination of a halogen lamp as the illumination source and a CdS member as the photosensitive member. A halogen lamp is generally used at a filament temperature of about 3000.degree. K. At this working temperature, the maximum value of its emission energy is in the infrared region of 800 to 900 nm and decreases gradually and constantly toward the shorter wavelength side. The spectral sensitivity of the CdS photosensitive member is high in the region near the infrared region. Consequently, the amount of exposure in the region of red to infrared is excessively large as compared with that in other regions, namely blue, green etc. This brings about the problem that characters and patterns written in red on the original are copied very thinly. In the worst case, they can not be copied at all.
The same problem also arises when a combination of an Se photosensitive member and an illumination source whose emission energy increases in the short wavelength region is used. In this case, in contrast with the above case, characters and patterns written in blue on the original are too thinly copied.
Similar unfavourable phenomena are observed with the combination of an Se photosensitive member and a halogen lamp and with the combination of a CdS photosensitive member and an illuminant whose emission energy becomes high in the short wavelength region.
This problem has not yet been solved although it is desirable to obtain a copy which has the same intensity distribution as that of the original. Obviously, the problem may be solved by compensating for the difference between the relative visibility and the spectral sensitivity mentioned above (such spectral sensitivity will also be referred to, hereinafter, as color sensitivity).
Two methods have already been proposed and used to solve the above problem. One of the known methods is to use a planar color filter. The other method is to use a multi-layer interference film which is disclosed, for example, in Japanese Patent Application Laid-Open No. 60142/1977 and Japanese Utility Model Application Laid-Open No. 99331/1977. These methods have been proposed to attenuate the light in a selected wavelength region thereby preventing over-exposure at the wavelength region. For example, in the case of the above-mentioned combination of a CdS photosensitive member and a halogen lamp, the light in the region near the infrared region is selectively decreased.
However, these prior art methods have some drawbacks.
The use of a planar color filter involves the problem of aberration in a parallel plane. As the color filter is added to the apparatus, the manufacturing cost of the apparatus increases. Further, a larger loss of light is caused by the surface reflection of the filter.
The second mentioned method employing a multi-layer interference film also leads to the problem of increased cost. Usually such interference film comprises many layers which are formed employing a very expensive technique such as vapour deposition. The spectral characteristics of the multi-layer interference film is not constant but variable depending on various factors of the optical system, in particular, depending on the incident angle of light. Further, the performance thereof is easily affected by heat and moisture and the film lacks durability. These are important drawbacks of the multi-layer interference film.
On the other hand, thermal ray absorbing filters are known in the art which attenuate the light in the near infrared region. Recently it has been proposed to use such filter as a condenser lens in a slide projector. However, it is by no means relevant to the subject of the present invention. The objective for which the thermal ray absorbing filter is used in a slide projector, is solely to prevent the elevation of temperature in the projector. It can never suggest any features of the present invention. According to the invention, the selection of wavelength is made from a broad range of wavelength regions after considering the spectral sensitivity of the photosensitive member being used. The prior art relates to the use of a thermal ray absorbing filter as a mere condenser for use in a slide projector. It has never been used as an image-forming lens which is required to have certain determined optical performance.